Water circulation unit

ABSTRACT

A water circulation unit ( 120 ) for reducing bacteria growth in a water supply system includes elements for controlling temperature and flow ( 133 ) which is arranged to provide a dynamically controlled flow of water from both a hot water pipe ( 140 ) and a cold water pipe ( 150 ) to at least one return water pipe ( 160 ) such that the water from the hot water pipe is above the temperature region in which dangerous bacteria growth may occur, and the water from the cold water pipe ( 150 )

The present invention relates to a water circulation unit and a tapwater network effectively impeding bacterial growth in the waternetwork. In particular, the present invention relates to a watercirculation unit adapted for mounting to or adjacent to a water tappingpoint.

BACKGROUND OF THE INVENTION

Growth of water-borne bacteria in supply water networks and tappingpoints is a well recognized problem. Especially feared are thelegionella bacteria that can grow rapidly to dangerous levels in thewater at temperatures between 20 and 50° C. Hot water flushing or abiocide may effectively kill free-living bacteria in the water, but inthe biofilm on the inner surfaces of water tanks, pipes and fittings thebacteria will often survive and start to multiply again as soon asconditions allow it. This capability of “hiding” makes certain bacteriae.g. legionella very difficult to effectively control. The problem isparticularly serious in large buildings, such as hospitals, hotels andapartment buildings, the tap water systems of which have many spaces inwhich at times stagnant water is cooled down or warmed up to thehazardous temperature region between 20 and 50° C.

Recent attempts to address this problem have included mixers where anoperator can use a special tool to temporarily flush the mixer and thewhole water supply system with scalding hot water for several minuteswith the aim of killing the bacteria during a round of all the tappingpoints in a hospital or care establishment, for example. Acomputer-controlled automated system for the regular flushing of tapwater systems with hot water through the tapping devices has also beensuggested. Further experience has shown that sanitizing effects areachieved by continuously supplying the water with oxidising biocidessuch as chlorine, bromine or ozone. Flushing methods are taught in e.g.U.S. Pat. No. 6,027,572 and references therein. However, all suchprocedures imply a great deal of manual labour and consequently highcosts for the service operator in, for example, a hospital. In addition,the activities in the building will be disrupted.

Other attempts of addressing the bacteria problem is by purifying thewater in, or close to, the tapping device. Methods and devices includeadding ozone (U.S. Pat. No. 5,942,125), using multiple filters (U.S.Pat. No. 5,851,388), adding bactericidal agent through a pumping device(U.S. Pat. No. 5,709,546) and sterilization by UV radiation (U.S. Pat.No. 5,891,329). Although effective in certain applications e.g. indental units, their complexity and need for maintenance make them lesssuitable for large-scale installation such as every tapping device in ahospital or an apartment building. Also the principle of purifying waterat a late stage and not addressing the problem of the growth of bacteriain the whole tap water system could be questioned.

In U.S. Pat. No. 6,021,803, by the same applicant as in the presentinvention the problem of legionella bacteria is addressed by providing atapping point including a mixer for hot and cold water, with a hot waterand a cold water inlet, and a hot water and a cold water space. Toimpede the growth of especially legionella bacteria within the mixer itis suggested that the mixer additionally is provided with a hot wateroutlet from the hot water space of the mixer. The outlet is connected toa hot water return pipe and through an arrangement of valves the hotwater is always kept under circulation. This will assure that the waterwill not cool down to the hazardous temperature region between 20 and50° C. Thermally insulated return pipes for hot water are, in fact,normally already installed as a part of the water mains of largebuildings, whereby in such cases, return pipes only need to be furnishedfrom each tapping point to the main return pipe. This will keep theinstallation and maintenance cost at a reasonable level. The cold watermay be circulated by a similar arrangement comprising a cold waterreturn pipe. Spaces within the tapping point, in which it is notpossible to circulate water, are evacuated whenever the tapping point isnot in use.

Swedish patent nr 517,749 teaches a method and apparatus of reducingbacterial growth in a water mixer by utilizing the method of U.S. Pat.No. 6,021,803 on the hot water side in combination with a device locallycirculating the water on the cold water side. If required the cold wateris additionally cooled by some cooling device. The additional cooling iscontrolled by a temperature sensor.

The water mixer of Swedish patent nr 517,749 will, in comparison withtraditional water mixers/tapping points, be complicated and typicallyrequire regular maintenance. The equipment will therefore be costly bothto install and maintain. The temperature sensor and the cooling devicewill typically require electricity and/or gas. An electricalinstallation, for example, has obvious drawbacks in for example ashower.

Thus there is a demand for methods and apparatus for preventing bacteriagrowth that can work with conventional water mixers/tapping points aswell existing supply water networks.

SUMMARY OF THE INVENTION

The objective problem is to provide a method and an apparatus forreducing the risk of bacteria growth, especially legionella bacteriagrowth, in a water supply network. In particular to provide an apparatusand a method that can be used with existing water supply networks andconventional water mixers/tapping points.

The problem is solved by the apparatus as defined in claim 1 and themethod as defined in claim 8.

In order to achieve the above-mentioned object, according to theinvention, a water circulation unit is provided that allows circulationof hot water and circulation of cold water. By circulating the hot andcold water, respectively, all parts of the system may be kept attemperatures above or below, respectively, the temperature region inwhich bacteria growth occur. In addition the return water may be kept ata temperature outside the dangerous region.

A realization of the water circulation unit according to the inventioncomprises means for controlling temperature and flow which are arrangedto provide dynamically controlled flows of water from both a hot waterpipe and a cold water pipe to at least one return water pipe such thatthe water from the hot water pipe is above a first predefinedtemperature and the water from the cold water pipe is below a secondpredefined temperature. The dynamic control should be based ontemperature. The means for controlling temperature and flow maypreferably be thermostatically controlled valves or a thermostat.

Thanks to the inventive water circulation unit it is possible toconstruct a water network with continuous circulation of hot and coldwater in all parts of the network. Parts not suitable to circulate withhot or cold water could be evacuated and ventilated.

Thanks to the inventive water circulation unit it is possible tosanitize cold water parts of the water supply system by flushing theseparts with hot water.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to thedrawing figures, in which

FIG. 1 a) is a schematic view of the water circulation unit according tothe invention, b) the water circulation unit according to the inventionmounted between a thermostatic water mixer and the water supply network,and c) the water circulation unit according to the invention between apair of individual tapping points for hot and cold water, and the watersupply network;

FIG. 2 a) is a cross sectional view of a preferred embodiment of thewater circulation unit according to the invention, and b) a crosssectional view of a further embodiment of the water circulation unitaccording to the invention;

FIG. 3 is a cross sectional view of yet another embodiment of the watercirculation unit according to the invention; and

FIG. 4 is a partly cross sectional view of an embodiment of the watercirculation unit provided within a mixer housing.

FIG. 5 a) is a schematic view of the water circulation unit according toone embodiment of the invention, and b) is a cross sectional view of theembodiment.

DETAILED DESCRIPTION OF THE INVENTION

The principle of the present invention will be described with referenceto the schematic illustration of FIG. 1 a. The Water Circulation Unit(WCU) 120 according to the present invention is installed in a watersupply system and provides circulation of hot water (hw) from a hotwater pipe 140 to a return water pipe 160 and circulation of cold water(cw) from a cold water pipe 150 to the same return water pipe 160. Meansfor controlling temperature and flow 133 of the return water controlsthe flow of water from the cold and hot water pipes 150, 140 to thereturn water pipe 160. The means for controlling temperature and flow133 should be adjusted to always give a flow of water both from the hotwater pipe 140 and the cold water pipe 150. By keeping a continuous flowof both hot and cold water all the way up to and through the WCU,bacteria growth is reduced by that the hot water will not cool down tothe dangerous temperature region, and the cold water will not heat up tothe dangerous temperature region. The arrows in the drawing indicatesthe flow of water from the hot and cold water pipe, respectively to thereturn water pipe. The means for controlling temperature and flow 133 istypically adjusted to give a larger flow of hot water (larger arrows)and a smaller flow of cold (smaller arrows). The temperatures arepreferably adjusted so that the temperature of the return water isbetween the temperature of the hot and cold water, but closer to thetemperature of the hot water. If, for example the hot water is 60° C.and the cold water is 10° C., an appropriate temperature for the returnwater would be around 57° C.

The principle of the invention is realized in the WCU depicted in theschematic drawing of FIG. 1 b. In FIG. 1 b a water mixer or tappingpoint 100, comprising a hot water inlet 105, a cold water inlet 110 anda mixed water outlet 115 leading to a shower (not shown), for example,is connected to the water circulation unit 120 according to the presentinvention. The water circulation unit comprises a hot water outlet (WCUhot outlet) 125 which is connected to the hot water inlet 105 of thewater mixer and a cold water outlet (WCU cold outlet) 130 connected tothe cold water inlet 110 of the water mixer. The water circulation unit120 further comprises a hot water inlet (WCU hot inlet) 135 connected toa hot water pipe 140, a cold water inlet (WCU cold inlet) 145 connectedto a cold water pipe 150 and a return water outlet (WCU return outlet)155 connected to a return water pipe 160. The cold water, hot water andreturn water pipes are connected to cold water, hot water and returnwater mains, respectively (not shown).

The water circulation unit 120 according to the invention branches off acontrolled portion of the water transported in the hot water pipe 140,the cold water pipe 150, or both, through the WCU return outlet 155 intothe return water pipe. By that the water in both the hot water pipes andthe cold water pipes can be kept outside of the temperature region(between 20 and 50° C.), in which dangerous bacteria growth may occur,even if the water mixer 100 is not in active use.

The water mixer 100 may be any kind of conventional water mixer ortapping device, for example a thermostat shower mixer. Preferably themixer is constructed for minimizing bacteria growth. Such mixers arecommercially available from for example AB Gustavsberg, Gustavsberg,Sweden, and relies upon the principle that spaces within the mixer areevacuated from water when the mixer is not in active use. The hot waterpipe 140, and the cold water pipe 150 are the feeding pipes forsupplying hot and cold water typically already installed and withstandardized dimensions, water pressure etc. Large buildings normallyhave hot return water pipes in the mains, but not connections to them atthe location of the tapping points.

Thus return water pipes from the location of the tapping points to themains typically need to be installed. The purpose of the return watersystem has primarily been to keep the hot water in the mains undercirculation and hence, keep the water in the mains hot, in order to beable to provide hot water at a tapping point at a reasonable time periodafter starting to flush the hot water. The hot water in the mains iskept at a predefined temperature, typically 50-55° C.

If WCUs according to the invention are installed in the water system acontinuous addition of water to the return water system will occur i.e.from the circulation of the cold water. This addition of water might attimes temporarily exceed the consumption of hot water in the system, andthe excess water is then preferably kept in a storage tank, therebybalancing the fluctuations in supply and demand. The storage tank ispreferably provided with a heater to ensure that the water temperatureis not falling below a predetermined level. Alternatively, the excesswater may be used in other facilities for example exchanging the waterin a swimming pool.

However, the amount of excess water i.e. the flow of cold water in theindividual WCU, should be kept at a minimum, in order to not putunnecessary high load on the return water system.

The return water should have a temperature outside the dangerous region,preferably well above, to prevent the bacteria growth. This put furtherrequirements on the flow of water and under normal conditions a smallflow of cold water is preferable.

To fulfil these demands, the means for controlling temperature and flow133 of the WCU according to the invention, should be able to at the sametime ensure a flow of hot water, a sufficient, but limited flow of coldwater and keep the temperature of the return water at a essentiallyconstant level well outside of the dangerous temperature region.

The water circulation unit 120 can for example be designed to be mounteddirectly adjacent to the water mixer, included in the wall-support thattypically fixes a shower mixer to the wall, built-in into the wall. Suchvariations should be apparent to the skilled in the art.

The water circulation unit 120 according to the present invention hashere been described mounted to a thermostat water mixer. This should beconsidered as a non-limiting example of the use of the presentinvention. The water circulation unit 120 may advantageously be used incombination with other types of water mixers and water faucets. Oneexample thereof is illustrated in FIG. 1 c, in which the watercirculation unit 120 is connected to a pair of individual faucets, forhot 165 and cold 170 water, respectively. An arrangement typically foundin traditional bathtubs and sinks. The water circulation unit 120 couldin this example be placed beneath the frame of the bathtub or sink.

A preferred embodiment of the water circulation unit according to theinvention is schematically depicted in FIG. 2 a. A hot water channel 205connects the WCU hot inlet 135 to the WCU hot outlet 125. A hot waterpassage 210 branches of from the hot water channel 205 and connects tothe means for controlling temperature and flow 133 which in turn isconnected to the WCU return outlet 155. Similarly a cold water channel235 connects the WCU cold inlet 145 to the WCU cold outlet 130. A coldwater passage 240 branches off from the cold water channel 235 andconnects to the means for controlling temperature and flow 133. Themeans for controlling temperature and flow 133 is here realized by athermostat valve 250 and optionally flow adjustment valve 255. Thethermostat valve 250 is preferably of the same type of valve thatcontrols the temperature of the outgoing water in a thermostat mixeri.e. automatically adjust the volume of hot and cold water to give theoutgoing water an essentially constant temperature corresponding to thesettings of the thermostat valve. The thermostat valves may have onepredefined temperature of the outgoing water or a possibility to set thetemperature. Such thermostat valves are commercially available in largevariety of shapes, sizes, temperature regions etc.

The thermostat valve 250 will ensure that the temperature of the waterin the return water outlet 155 is essentially constant by increasing theflow of warm water from the hot water channel 205 via the hot waterpassage 210 if the return water is too cool, and increase the flow ofcold water from the cold water channel 235 via the cold water passage240 if the return water is too warm. The water flow adjustment valve 255will define a maximum flow of water in the return water outlet 155.Alternatively, the hot and cold water passages may be provided withindividual flow adjustment valves, which maximize the flow of hot andcold water, respectively, and in combination define a maximum total flowof water in the return water outlet 160.

The thermostat valve 250 also has to ensure that the hot water is alwaysabove, and the cold water is always below the dangerous temperatureregion, i.e. ensure a continuous and sufficient flow of both hot andcold water. This is achieved by setting the temperature of thethermostat valve 250 at an appropriate value, or choosing an appropriatevalve if a thermostat valve with a fix temperature setting is used,compared to the temperatures of the hot and cold water. By setting(choosing) the thermostat valve 250 to a temperature between thetemperature of the hot and cold water, but closer to the temperature ofthe hot water, the flow of hot water will in most instances be largerthan the flow of cold water, which is according to the above statedpreferences. Since the hot water in the supply system is subjected toreheating and circulation in the mains the temperature of the hot waterwill be roughly constant. It is recommended to keep the temperature ofthe hot water around 60° C. The temperature of the supplied cold waterwill typically vary more than the supplied hot water. The variationsarising from the level of consumption of cold water in the system andoutside factors such as the surrounding temperature. The cold water isfor example often considerably cooler a cool winter day than a hotsummer day. The thermostat valve 250 will account for the fluctuationsby increasing the flow of cold water if the cold water is “warm” anddecrease the flow if it is “cold”. This is in accordance with therequirements of both keeping the temperature of the cold water below thedangerous temperature region and to limit the flow of cold water.

If, for example the hot water is 60° C. and the cold water is 10° C., anappropriate setting for the thermostat valve 250 would be around 57° C.The flow of hot water will be larger than the flow of cold water, whichis desirable to keep the need of storage volume at an acceptable level,but circulation of cold water will be sufficient. The setting of thethermostat valve 250, or the choice of valve if valves with fixedtemperature are used, and the adjustment of the flow adjustment valves255, 265 will depend on the implementation, for example the temperaturesof the hot and cold supply water, the facilities for temporary storingetc. The settings can for example be preset by the manufacturer and ifneeded adjusted during installation or regular maintenance. However, thesettings should not be changed during normal operation.

The means for controlling temperature and flow 133 may be realized inother ways than the above described. In a further embodiment of theinvention, depicted in FIG. 2 b, the means for controlling temperatureand flow 133 is accomplished by an arrangement of valves which comprisehot and cold flow adjustment valves 255, 265 and hot and coldthermostatically controlled valve 250, 260, arranged in the respectivehot and cold water passages 210, 240. The hot water thermostaticallycontrolled valve 250, for example a bi-metal valve, is arranged to beclosed if the temperature of the water in the hot water passage 210 isabove a predefined threshold value, the hot water threshold value, andopen if the temperature of the water is below the hot water thresholdvalue. The hot water threshold value should be over the upper value ofthe temperature region for hazardous bacteria growth, typically 60°C.±3° C. The hot water adjustment valve 255 defines the maximum flow ofwater in the hot water passage 210, and is preferably adjustable fromthe outside of the water circulation unit 120.

The cold water thermostatically controlled valve 260, for example abi-metal valve, is arranged to be closed if the temperature of the waterin the cold water passage is below a predefined threshold value, thecold water threshold value, and open if the temperature of the water isabove the cold water threshold value. The cold water threshold valuemust be lower than the lowest value of the temperature region forhazardous bacteria growth, typically 16° C. ±2° C. The cold wateradjustment valve 255 defines the maximum flow of water in the cold waterpassage 240, and is preferably adjustable from the outside of the watercirculation unit 120.

Flow adjustment valves and thermostatically controlled valves suitablefor the present invention are commercially available in a large numberof variations. As should be apparent for the skilled in art, the detailsof the valve arrangements can be varied and still be within the scoop ofthe invention.

If the water in the hot water passage 210 cools down to a temperaturebelow the hot water threshold, typically due to that the water mixer 100has not been in use for a time period, the hot water thermostaticallycontrolled valve 250 will open. A circulation of hot water from the hotwater pipe 140 through the hot water passage 210, to the return waterpipe 160 will occur. The temperature of the water and the surroundingmaterial will rise to, or above, the hot water threshold value and thehot water thermostatically controlled valve 250 will then close again.The inherent hysteresis and slowness of for example a bi-metal valvewill prevent the valve from rapidly switching on and off, andcirculation will occur for a significant period of time. The hot waterthreshold value should be chosen so that the temperature of the hotwater never is in the hazardous region and all fluctuations of the watertemperature are above the hazardous region.

Similarly, If the water in the cold water passage 240 warms up to atemperature above the cold water threshold, typically due to that thewater mixer 100 has not been in use for a time period, the cold waterthermostatically controlled valve 260 will open. A circulation of coldwater from the cold water pipe 150 through the cold water passage 240,to the return water pipe 160 will occur. The temperature of the waterand the surrounding material will cool to the cold water threshold valueand the cold water thermostatically controlled valve 260 will then closeagain. The inherent hysteresis and slowness of for example a bi-metalvalve will prevent the valve from rapidly switching on and off, andcirculation will occur for a significant period of time. The cold waterthreshold value should be chosen so that the temperature of the coldwater never is in the hazardous region and all fluctuations of the watertemperature are below the hazardous region. The thermostaticallycontrolled valves 260, will through the heat transfer in the material ofthe WCU 120 affects each other and a reasonably steady state situationwill occur in which a continuous flow of both hot and cold water may beachieved.

In order to effectively impede bacterial growth, water standing still,and hence cooling down or warming up to the hazardous temperatureregion, should be carefully avoided. The water circulation unitaccording to the invention should therefore preferably be combined witha water mixer/tapping point that automatically evacuate all water spacesthen not in use, or disinfect its interior with other means. To enhancethe circulation in the spaces formed between the water passages 210, 240entrance to the water channels 205, 235 and water mixer inlets 105, 110,the distance between the water passages 210, 240 entrance to the waterchannels 205, 235 and water mixer inlets 105, 110 should be as short aspossible. Preferably the water mixer/tapping points has valves closingoff the cold and hot inlet water as close as possible to the inlets 105,110, which are connected to the water circulation unit outlets 125, 130.

In one embodiment of the present invention, described with reference toFIG. 3, the circulation close to the water mixer is further enhanced.The hot water passage 210 is in this embodiment joined to a hot watercirculation tube, 300, extending from the hot water passage 210 into thehot water channel 205 towards the WCU hot outlet 125. In the same mannerthe cold water passage 240 is joined to a cold water circulation tube,305, extending from the cold water passage 240 into the hot waterchannel 205 towards the WCU cold outlet 130. The circulation of waterwill by this arrangement be effective in all parts of the water channels205, 235. If suitable, with regards to the design of the water mixer,the circulations tubes 300, 305 may extend beyond the water circulationunit outlets 125, 130 and a distance into the water mixer inlets 105,110. The appropriate distance will depend on the design of the watermixer, and may readily be adapted, for example by cutting the tubes inappropriate lengths, upon installation of the water circulation unit.Other types of “tube in tube” arrangement could be used in the samemanner. For example may the parts of the water channels from the waterpassages and towards the WCU outlet be longitudinally divided into twoparts.

By this arrangement the water circulation unit 120 according to theinvention will provide circulation, and hence a possibility to avoid thehazardous temperature region also very close to the water mixer/tappingpoint to which the water circulation unit is connected. The enlargedsection of FIG. 3 illustrates, with arrows, how water will flow aroundthe end of the circulation tubes.

The principles of the present invention can be utilized also within awater mixer or tapping point by incorporating the WCU 120 in the mixerhousing and optionally integrate it with the design of the water mixer.This embodiment of the invention is illustrated in FIG. 4 in which awater mixer 400 comprising a hot water chamber 410, a cold water chamber415 and a mixer chamber 405 is depicted. As illustrated the hot and coldwater passages 210 and 240 are provided in the wall of the mixerhousing. The thermostat valve 250 is as before connected to the hot andcold water passages and the flow adjustment valve 255 at the returnwater outlet 155. The hot and cold water passages 210, 240 may as beforebe connected to the water channels 205, 235, but may preferably beconnected to the hot and cold water chamber, respectively, in order toprovide circulation also in these chambers. The functionality of thethermostat valve 250 will be in accordance with previous embodiments.

The invention has in the foregoing embodiments been described with themeans for controlling temperature and flow 133 comprising purelymechanical members, utilizing the physical effect of thermal expansionto control the temperature of the outgoing return water. These arepreferred embodiments since such systems does not require any electricalpower and generally very little or no maintenance. However, asappreciated by the skilled in the art, means for controlling temperatureand flow 133 may also be realized in other ways, for example by the useof a sensor for measuring the temperature, electrically powered flowcontrol valves and control electronics for adjusting the valves inresponse to the measuring of the sensor.

The cooling of the cold parts of the water supply system afforded by theinvention will effectively reduce the bacteria growth, but the coolingwill not kill the bacteria. In the hot parts, if the water issufficiently hot, the bacteria will be killed. If needed, thanks to thepresent invention, also the cold parts may be flushed with hot water bytemporarily couple hot water to the cold water system and optionallycold water to the hot water system. Conventional thermostat valves willreact to the reversed temperature situation by allowing maximum flowfrom the cold water passage (now with hot water). In the embodimentutilizing bi-metal valves the effect will be the same. This will providefor a effective sanitation of the cold water parts. Some thermostatvalves are not sensitive to which connection the hot and cold water isconnected to and hence, will function equally well with reversed coldand hot water. This inventive method of sanitizing is particularlyuseful before opening an establishment that has been temporarily closed,for example a season hotel.

In the here described preferred embodiment the hot water passage 210 andthe cold water passage 240 are connected to the same return water pipe.They could equally well be connected to individual return water pipes,in which case the return water typically should be re-cooled before itis brought back to the cold water supply mains. This is often the casein for example very tall buildings, wherein the cold water is circulatedin the mains in the same manner as the more common hot watercirculation. The water circulation unit according to the invention caneasily be adapted to function also in such systems. Such as anadaptation is schematically depicted in FIG. 5 a, and a realization of aWCU suitable for individual hot and cold water return pipes areillustrated in FIG. 5 b.

Illustrated in FIG. 5 a is the water circulation Unit (WCU) 520according to this embodiment of the present invention, installed in awater supply system and provides circulation of hot water (hw) from ahot water pipe 140 to a hot water return pipe 560 h and circulation ofcold water (cw) from a cold water pipe 150 to a cold water return pipe560 c. Means for controlling temperature and flow 533 h and 533 c, forthe hot and cold water, respectively, of the return water, controls theflow of water from the cold and hot water pipes 150, 140 to the returnwater pipes 560 h and 560 c, respectively. The means for controllingtemperature and flow 533 h and 533 c should preferably dynamicallyadjust the flow of water from the hot water pipe 140 and the cold waterpipe 150, to ensure that the temperature of the hot water never fallsbelow a predefined hot water value, respectively to ensure that thetemperature of the cold water never exceeds a predefined cold watervalue. The bacteria growth is reduced by that the hot water will notcool down to the dangerous temperature region, and the cold water willnot heat up to the dangerous temperature region. The arrows in thedrawing indicates the flow of water from the hot and cold water pipe,respectively to the respective return water pipes.

A realization of this embodiment of the present embodiment invention isschematically depicted in FIG. 5 b. A hot water channel 205 connects theWCU hot inlet 135 to the WCU hot outlet 125. A hot water passage 210branches of from the hot water channel 205 and connects to hot watermeans for controlling temperature and flow 533 h which in turn isconnected to the hot water return outlet 155. Similarly a cold waterchannel 235 connects the WCU cold inlet 145 to the WCU cold outlet 550.A cold water passage 240 branches off from the cold water channel 235and connects to cold water means for controlling temperature and flow533 c. The means for controlling temperature and flow 533 c and 533 hcomprises, for example, an arrangement of valves which comprise hot andcold flow adjustment valves 555, 565 and hot and cold thermostaticallycontrolled valves 550, 560, arranged in the respective hot and coldwater passages 210, 240. The hot water thermostatically controlled valve250, for example a bi-metal valve, is arranged to be closed, or toprovided a small flow, if the temperature of the water in the hot waterpassage 210 is above a predefined threshold value, the hot waterthreshold value, and to open up if the temperature of the water fallsbelow the hot water threshold value. The hot water threshold valueshould preferably be over the upper value of the temperature region forhazardous bacteria growth, typically 60° C.±3° C. The hot wateradjustment valve 555 defines the maximum flow of water in the hot waterpassage 210, and is preferably adjustable from the outside of the watercirculation unit 520.

The cold water thermostatically controlled valve 560, for example abi-metal valve, is arranged to be closed, or to provided a small flow,if the temperature of the water in the cold water passage is below apredefined threshold value, the cold water threshold value, and open upif the temperature of the water is above the cold water threshold value.The cold water threshold value must be lower than the lowest value ofthe temperature region for hazardous bacteria growth, typically 16°C.±2° C. The cold water adjustment valve 565 defines the maximum flow ofwater in the cold water passage 240, and is preferably adjustable fromthe outside of the water circulation unit 520. Hence, a dynamic control,based on water temperature, of the flow and/or the temperature of boththe hot and cold water is achieved, which ensures that regardless offluctuations of the temperature of the incoming water (within obviouslimits), both the hot and cold water will be dynamically andautomatically adjusted to in their respective “safe” temperature region.At the same time the flows of water to the return pipes are minimized,keeping water and/or energy consumption at a low level.

Dynamic control based on temperature may, as appreciated by the skilledin the art, be implemented in many different ways, including forexample, the above described thermostatically controlled valves,different types of thermostats, systems comprising electronic sensorsand electrically manoeuvred valves.

The water circulation unit 520 suitable for separated cold water return560 and hot water return, has here above been depicted and described asa combined unit comprising both hot and cold water parts. Of course, thehot water circulation and the cold water circulation may be provided intwo separate units, which can be mounted independently, preferably inclose proximity to the mixer/faucets. This could be advantageous incertain applications, for example if separate faucets are used for hotand cold water, or if only a cold water, or a hot water faucet, is usedat one location. Further, if for example, the mains provides hot waterreturn only, separated water circulation units for hot water onlyaccording to the above embodiment, may advantageously be installed.Alternatively, if only cold water return is available, separated watercirculation units for cold water only may be installed.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

1-9. (canceled)
 10. A water circulation unit for reducing bacteriagrowth in a water supply system adapted for installation in proximity toa tapping point and in connection to a hot water pipe, a cold water pipeand a return water pipe, wherein the water circulation unit comprises: ahot water channel leading from the hot water pipe to the tapping point,and a hot water passage which branches off from the hot water channeland connects to the return water pipe; a cold water channel leading fromthe cold water pipe to the tapping point and a cold water passage whichbranches off from the cold water channel and connects to the returnwater pipe; whereby the hot and cold water passages are adapted forproviding circulation of water from the hot and cold water pipes to thereturn water pipe; and means for dynamically controlling of temperatureand flow which automatically controls flow of water from the hot waterpipe and the cold water pipe to at least one return water pipe, saidautomated control arranged to be dependent at least on the watertemperature in the return water pipe.
 11. The water circulation unitaccording to claim 10, wherein said automated control is arranged to bedependent on the water temperature in the return water pipe and thewater temperatures in the hot water passage and cold water passage. 12.The water circulation unit according to claim 10, wherein said automatedcontrol is arranged to be dependent on the water temperature in thereturn water pipe and the water temperature in the hot water passage.13. The water circulation unit according to claim 10, wherein saidautomated control is arranged to be dependent on the water temperaturein the return water pipe and the water temperature in the cold waterpassage.
 14. The water circulation unit according to claim 11, whereinthe means for controlling temperature and flow is arranged to providethat the water from the hot water pipe is above a first predefinedtemperature and the water from the cold water pipe is below a secondpredefined temperature, and that the temperature of the water in thereturn water pipe is at or above a third predefined temperature.
 15. Thewater circulation unit according to claim 14, wherein the thirdpredefined temperature provided by the means for controlling flow andtemperature is arranged to be in-between the first predefinedtemperature and the second predefined temperature.
 16. The watercirculation unit according to claim 15, wherein the third predefinedtemperature provided by the means for controlling flow and temperatureis arranged to be closer to the first predefined temperature than thesecond predefined temperature.
 17. The water circulation unit accordingto claim 10, wherein the means for controlling temperature and flowcomprises a thermostat vale.
 18. The water circulation unit according toclaim 11 wherein the water circulation unit comprises: a first hot watervalve provided in the hot water passage; and a first cold water valveprovided in the cold water passage, wherein the first hot water valve isarranged to be open if the temperature of the water in the hot waterpassage is below the first predefined temperature, and the first coldwater valve in the cold water passage is arranged to be open if thetemperature of the water in the cold water passage is above the secondpredefined temperature.
 19. A method in a water supply system forreducing bacteria growth in the water supply system, comprising: leadinga portion of the hot water passing in a hot water channel leading from ahot water pipe to a tapping point, via a hot water passage to a returnwater pipe; leading a portion of the cold water passing in a cold waterchannel from a cold water pipe to the tapping point, via a cold waterpassage to the return water pipe; whereby providing for circulation ofwater from the hot and cold water pipes to the return water pipe; anddynamically controlling the flow of water from the hot water pipe andthe cold water pipe to the return water pipe, so that the temperature ofthe water in the return water pipe is above a predefined value.
 20. Themethod according to claim 19, wherein in the step of controlling theflow of water the water from the hot water pipe is above a firstpredefined temperature, the water from the cold water pipe is below asecond predefined temperature, and that the temperature of the water inthe return water pipe is at or above a third predefined temperature. 21.A method in a water supply system for reducing bacteria growth in thewater supply system, wherein the system utilizes means for circulationof both cold and hot supply water, the method comprises a step ofleading hot water into cold water supply mains and whereby sanitizingbacteria in parts of the supply water system that is cold under normaloperation.